Nitrated aromatic hydrocarbons and nitrated halogenated aromatic hydrocarbons and particularly mononitrobenzene are important chemical intermediates. Guenkel et al, U.S. Pat. No. 5,313,009 (the '009 patent), assigned to the assignee of the present invention, describes a continuous process to nitrate a nitratable aromatic compound, typically an aromatic hydrocarbon or a halogenated aromatic hydrocarbon, and particularly benzene, in which the formation of oxidation by-products is substantially reduced compared with the prior art and wherein the reaction rate is substantially increased. The disclosure of this patent, in its entirety, is incorporated herein by reference. According to the process of the '009 patent, NO.sub.x gases are still formed which preferably are not released into the atmosphere for environmental reasons. The prior art has suggested treating these NO.sub.x gases with a caustic solution to form sodium nitrite or sodium nitrate salts, which are disposed of as the salt. Small quantities of CO.sub.2 are also formed in the nitration process. CO.sub.2 will react with sodium hydroxide to form sodium carbonate; this increases the consumption of sodium hydroxide. Further, the use of sodium hydroxide and formation of salts which must be subsequently disposed of is not completely acceptable on an industrial basis. The discharge of nitrates and especially nitrites of sodium is facing increasingly strict regulation.
The present invention describes a system for removal of NO.sub.x gases which can be utilized in conjunction with the nitration process disclosed in the aforesaid '009 patent, as well as other nitration processes where NO.sub.x gases are formed to efficiently and effectively convert the NO.sub.x gases into weak nitric acid, preferably recycled to the nitric acid feed stock.